4.2 High-Performance Liquid Chromatography and Ion Chromatography

High-Performance Liquid Chromatography and Ion Chromatography

Gas chromatography can only analyse what will vaporise intact. A large and growing fraction of workplace contaminants will not: isocyanates polymerise, formaldehyde is too reactive and volatile to trap conventionally, hexavalent chromium is an inorganic ion, and amines and acids are too polar for a standard GC column. These analytes are handled by liquid-phase chromatography, and the CIH exam expects you to know which technique a given analyte requires and why.

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1. The Selection Rule

   Is the analyte volatile AND thermally stable at 200-300 C?
                 |                              |
                YES                            NO
                 |                              |
        Gas Chromatography            Is it an ion or ionisable
        (GC-FID, GC-MS, GC-ECD)       inorganic species?
                                          |            |
                                         YES          NO
                                          |            |
                                  Ion Chromatography   HPLC
                                  (suppressed          (reversed-phase,
                                   conductivity)        UV/fluorescence)

The discriminator is volatility and thermal stability, not concentration or sample size. A candidate who reaches for GC because "it is the standard IH technique" will get isocyanate and chromate questions wrong.

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2. High-Performance Liquid Chromatography

In HPLC the mobile phase is a liquid pumped at high pressure (typically 1,000 to 6,000 psi) through a column packed with small (3–5 µm) particles. Separation depends on the analyte's relative affinity for the stationary and mobile phases.

Reversed-phase versus normal-phase

The name is historical: normal-phase came first, so the more useful configuration got called "reversed."

Isocratic versus gradient elution

• Isocratic: mobile phase composition held constant. Simple, reproducible, but poor at resolving mixtures with a wide polarity range.
• Gradient: organic fraction increased during the run (for example 20% to 90% acetonitrile). Sharpens late peaks and shortens run time, at the cost of column re-equilibration between injections.

HPLC detectors

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3. Ion Chromatography

Ion chromatography (IC) is HPLC configured for ionic species. The column carries fixed charged functional groups; analyte ions exchange with counter-ions and are separated by charge density and size.

The defining feature is suppressed conductivity detection. The eluent itself is ionic and would swamp a conductivity detector, so a suppressor placed between the column and the detector chemically converts the eluent to a low-conductivity form (for example, converting sodium hydroxide eluent to water) while converting analyte anions to their more conductive acid forms. The result is a large signal-to-background improvement.

Industrial hygiene applications of IC:

• Inorganic anions: fluoride, chloride, nitrate, nitrite, phosphate, sulphate
• Acid gases collected on treated filters or in impingers: HCl, HF, HNO₃, H₂SO₄
• Amines and quaternary ammonium compounds (cation exchange)
• Hexavalent chromium, which is the highest-stakes IC application in industrial hygiene

Hexavalent chromium: why the method matters

Cr(VI) is a Category 1 carcinogen with an OSHA PEL of 5 µg/m³ and an action level of 2.5 µg/m³. The analytical challenge is that Cr(VI) can be reduced to Cr(III) on the filter by co-collected iron, welding fume, or acidic conditions, producing a falsely low result.

The reference approach separates Cr(VI) by ion chromatography and detects it by post-column reaction with 1,5-diphenylcarbazide, forming an intensely coloured complex measured at about 540 nm. Because the chromatographic step separates Cr(VI) from Cr(III) before the colour reaction, the method reports only the hexavalent species. Sample handling matters as much as the instrument: alkaline extraction and prompt analysis limit inter-conversion.

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4. Practical Considerations the Exam Tests

• Derivatisation is a collection step, not a lab afterthought. Isocyanate sampling media are impregnated with the derivatising reagent so the reaction occurs at the moment of collection, before the isocyanate can polymerise. Formaldehyde is collected on 2,4-dinitrophenylhydrazine-coated media for the same reason.
• Mobile phase composition is part of the method. Substituting methanol for acetonitrile changes retention and can invalidate identification by retention time.
• HPLC is not inherently more sensitive than GC. Sensitivity depends on the detector; a fluorescence detector on a derivatised analyte can outperform GC-FID by orders of magnitude, while a refractive index detector is far worse.
• Holding times are method-specific. Derivatised isocyanate samples and Cr(VI) samples both have limited stability, and exceeding the holding time is a data quality defect that must be flagged, not silently ignored.